P0171

DTC P0171 Fuel Trim System Too Lean

System Description
To provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air/fuel metering system is used. While in Closed Loop, the Engine Control Module (ECM) monitors the Oxygen sensor 1 (O2S1) signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with the scan tool. Ideal fuel trim values are around 128 (0%). If the O2S1 signal is indicating a lean condition, the ECM will add fuel resulting in fuel trim values above 128 (0% to 100%). If a rich condition is detected, the fuel trim values will be below 128 (0% to -100%), indicating that the ECM is reducing the amount of fuel delivered. If exhaust emissions reach an excessive level due to a lean or rich condition, a fuel trim Diagnostic Trouble Code (DTC) is set.

Conditions for Setting the DTC
- The average of short term fuel trim value is greater than or equal to 0.9375.
- No intrusive tests active.
- DTCs P0106, P0107, P0108, P0112, P0113, P0115, P0116, P0117, P0118, P0122, P0123, P0126, P0131, P0132, P0133, P1133, P0134, P1134, P0135, P1167, P1171, P0201, P0202, P0203, P0204, P0217, P0300, P0336, P0337, P0341, P0342, P0351, P0352, P0402, P0404, P1404, P0405, P0406, P1441, P0443, P0506, and P0507 are not set.
- Throttle Position (TP) is less than 95%.
- Engine speed is between 700 and 6000 rpm.
- Barometric Pressure (BARO) is greater than 72.0 kPa (10.4 psi).
- Coolant temperature is between 70 °C (158 °F) and 115 °C (239 °F).
- Manifold Absolute Pressure (MAP) is between 25 kPa (3.6 psi) and 99.7 kPa (14.5 psi).
- Intake Air Temperature (IAT) is between -40 °C (-40 °F) and 120 °C (248 °F).
- Airflow is between 1.5 and 45 g/sec.
- Vehicle speed is less than 140 km/h (87 mph).
- System is in closed loop.
- Adaptive index is ready.
- System voltage is greater than 11 volts.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate after four consecutive ignitions in which the diagnostic runs with the fault active.
- The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.

Conditions for Clearing the MIL/DTC
- The MIL will turn off after four consecutive ignition cycles in which the diagnostic runs without a fault within the freeze frame conditions that the DTC failed.
- A history DTC will clear after 40 consecutive warm-up cycles without a fault.
- The DTC(s) can be cleared by using the scan tool.
- Disconnecting the ECM battery feed for more than 10 seconds.

Diagnostic Aids

CAUTION: After repairs, use the scan tool Fuel Trim Reset function to reset the long-term fuel trim to 128 (0%).

- Fuel pressure - The system will be lean if the pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm.
- Map sensor - An output that causes the ECM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the ECM to substitute a fixed (default) value for the MAP sensor. If the lean condition is gone when the sensor is disconnected, substitute a known good sensor and recheck.
- Fuel contamination - Water, in even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water causes a lean exhaust and can set DTC P0171.

Check for poor O2S1 or MAP sensor connection at the ECM. Inspect the harness connectors for the following conditions:
- Backed-out terminals
- Improper mating
- Broken locks
- Improperly formed
- Damaged terminals
- Poor terminal-to-wire connection

Inspect the wiring harness for damage. If the harness appears to be OK, observe the O2S1 display on the scan tool while moving the connectors and the wiring harness related to the engine harness. A change in the display will indicate the location of the fault. Check the brake power booster check valve for possible leaks.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
5. Visually/physically checking items which may cause a lean condition may determine the cause of the DTC being set and save diagnosis time.
9. A vacuum leak can change the Fuel Trim Index and set DTC P0171. This step checks the intake manifold for vacuum leaks.
11. Contaminants in fuel, such as alcohol or water, can create a lean condition setting DTC P0171. Checking for these contaminants could identify the malfunction.
17. If no faults have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.

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